'The biggest mercenary in football'

The 29-year-old Sanchez had widely been expected to join rivals Manchester City in the summer transfer window but a deal fell through on deadline day.

Despite City reviving their interest in January, and Chelsea inquiring about a deal, the 29-year-old opted to join Jose Mourinho in the red half of Manchester.

As the saga played out former Arsenal defender Martin Keown called Sanchez "the biggest mercenary in football," while another former Gunner Ian Wright said in December the Chilean had "clocked off."

It has been widely reported that Sanchez's weekly salary at United will range from $420,000 to $700,000.

On Instagram, Sanchez did not reference anyone directly as he defended his record at Arsenal, writing: "There are people (former club players) who have spoken with no knowledge of what happens inside the club and cause damage.

"I must say I always gave 100%, until the last day, when I asked to the Mister (Arsenal manager Arsene Wenger) to be in the team, because I wanted to be a contribution."

Mourinho believes Sanchez is "one of the best attacking players in the world."

"He will bring his ambition, drive and personality, qualities that make a Manchester United player and a player that makes the team stronger and the supporters proud of their club dimension and prestige," added the United manager.